The bearing contact angle is the angle between a plane perpendicular to the bearing axis and a line extending through the points of contact of a bearing ball with the raceways of the inner and outer rings. Measurement of this angle provides an extremely good check on the general geometrical accuracy of the various components in the bearing. It has also been established in certain applications that if a bearing is to function correctly for a predetermined operational life, then the contact angle must be within closely defined limits.
In order to measure the contact angle of a bearing a small axial load may be applied to the outer ring (with the inner ring held stationary). A special gauging tool rotates the outer ring so that it completes a predetermined number of revolutions, such as 20 revolutions, and simultaneously tracks revolutions of the retainer or ball set, i.e. it counts the number of rolling elements that pass a certain point for each revolution of the outer ring. This data, the retainer rev count and the outer race rev count, together with knowledge of the dimensions of the components of the rolling element bearing may then be used to calculate the initial bearing contact angle (the bearing contact angle at zero axial load).